Polyphase Troubleshooting

[Dave_B]

I've been struggling to get my Polyphase to come to life for a few days now.  Audio passes through it fine (tested with an audio probe), but it doesn't phase.  The main LFO isn't working yet, but the Envelope Follower section is, so I've been testing with some pulsating white noise.

Can someone explain to me how the voltage should be distributed across the section outlined in red?  I'm thinking the transistors could potentially go to ground and the LED's should be dropping around 5v total, leaving 10v/26ma across the 390ohm resistor.  That's great except that I've got 7 volts across the first LDR.  I'm guessing that suckers dead, and possibly the cause of my problem.

What does the 5087 do?  Is it part of the LFO?  I'm assuming the other 5088's are.  Someone please correct me if I'm wrong.



I've been looking at this too long to make heads or tails of it anymore.


edit: topic title changed at Dave's request.

[nelson]

Thats strange.

Have you checked the value of the decoupling resistor leading to the first LED?

Is the LFO functioning at all?

[Dave_B]

Well, here's the interesting thing: When you put the optos in correctly, the thing fires right up...     Somehow I was interpeting the markings incorrectly.  When I looked at the schematic with a clear head, a light bulb finally came on.

The important thing to reaffirm is that it isn't ticking at all.  It's out on the workbench and I wasn't careful about wire placement or shielding.  This output is shielded because I had a piece or rg-174 ready to go, but the input isn't. 

I've still got a couple of issues to resolve.  It distorts if I play full chords.  That's with a Super Distortion going straight in.  Also, the LFO on the Envelope Follower is a little too subtle.  I was expecting it to go over the top if I cranked it but it doesn't get there.  Does that sound normal?  These things I can handle.

One mod I'd like to add is an LED in series with the VTL5C3/2's to get some visual feedback.

Thanks again for the effort, Paul.  This is a great sounding circuit!

[lowstar]

hey dave,

great to hear you got it working.
mine´s already boxed in and the ticking no real problem, though it can be heard if you use headphones.

one thing bothers me still, though: the feedback pot introduces a pretty annoying hiss and swishing-swooshing sounds in some positions; from zero to 15%, there is loud hissing, then the next 10% are pretty ok, then for the next 50% of the pot rotation, the swishy-swooshy thing takes over in varying stages of loudness, the it becomes quieter for some 15% again, then for the end of the pot travel, it is annoying again.

now i know that the feedback pot is supposed to introduce some noise added to the signal on its way, but not that loud and not at the first 15% of the pot travel.

also, compared to sound samples on various sites, the hissing is too loud.

so, dave, does yours do the same thing ?
i think i still have a bug in mine.

cheers,
lowstar

[Dave_B]

I've not gotten to any critical listening tests yet, but I can tell you that I've not noticed any change in hiss as I adjust the feedback.  It's consistant no matter where I'm at in the rotation.  I do get oscillation toward the end of the rotation, but that looks normal.  It's actually a good noise maker when you feed percussive sounds into it.

Have you checked out the 4558's with an audio probe?  I threw one together when I started troubleshooting the Polyphase and it's a great thing to have. 

[nelson]

Its certainly not an easy build. 

To answer your questions: now I have an original. The effect on the envelope section should be quite pronounced.

I will open it up and take gut shots eventually.

[Dave_B]

Regarding the EF's Modulation LFO, I can hear it much better with white noise vs. a guitar, though it's still not knocking my socks off.  I noticed the Deluxe has an 820k on the Mod Level pot vs. the 620k on the Poly.  I'll play around with that value and report back. 

I found out a little about the thermistor but nothing good.  It was made by Fenwal, which is now owned by Honeywell.  There's no cross reference on their site and my emails have gone unanswered.

[lowstar]

UPDATE on my noise issue w/feedback pot:
went through each stage with an audioprobe, and, sure enough, one of the rc4558p´s was defective. replaced it with a ba4558 and most of the noise in general in the feedback path and the shhh on the first 10% pot rotation was gone. 
at the end of the pot travel, there was still a spot where it would come back again kind of abrupt, so i thought "ok, let´s switch them all for tl072 to bring down the noise floor". did it, but at the spot where the 4558p´s would bring back the noise, it now sounded like a mobile telephone trying to make a call...

so i swapped all the 5 opamps for ba4558 (actually cheap generic opamps), and...bingo ! now it even behaves in the last 10% of the feedback pot rotation, and the overall noise floor is also better.
a funny finding, i always thought those rc4558p´s were way better than ba4558...

and also, thumbs up for audioprobing. it saved my bad stone and now it saved my polyphase.

cheers,
lowstar

[StephenGiles]

Interesting, what manufacturer are the BA4558s? What about the thermistor - or have I missed something? Another question - how would VTL5C4/2s work here? I have a bunch I bought from Steve Daniels a while back, getting uncomfortable in their bag!

[nelson]

My production vintage polyphase is currently in storage, move into a GREAT new place on monday, and when I get a chance I will unpack it and verify a few key components, in particular the thermistor.

If anyone else has any suggestions on any component values they would like verified, post them in this thread. I did work from both the factory schema for the dlx phaser and Polyphase. I wont have time to do a complete Rev Eng for a while.


I will also make some sound samples with corresponding settings for you guys to compare your DIY builds to the real unit.

I havent even had the chance to open it up yet, but I am looking forward to looking under her skirt.

One thing I have noticed is the Env mod section sounds a lot more prominent on the original than it does on my build.

[Dave_B]

One thing I have noticed is the Env mod section sounds a lot more prominent on the original than it does on my build.

As you probably know, the Envelope Modulator voltage is added to the EF voltage through an 820k mixing resistor.  I could see it working correctly on the scope but like yours, the modulation didn't seem to be doing enough.  I swapped the 820k resistor for a 1meg pot and found that a much lower value, probably 150k-200k works a lot better.   Once I take it out and measure it, I'll post the value.

A couple of other things:
I'm pretty sure two of the pots are backwards on the board.  Assuming the schematic is accurate, Envelope Sensitivity and Modulation Rate are reversed.  If you're looking down at the component side of the board with the knobs along the top, the lugs should be wired:

Code Select Expand


Fdback  |  Rate   | Env Sens | Env Md  | Env Rate
1 2 3   |  1 2 3  |  3 2 1   |  1 2 3  |  3 2 1

Someone correct me if I've got it wrong, but mine responds a lot better since I changed them.

The Rate knob needs to be tweaked on mine.  When I turn the knob past four o'clock, the oscillator is either out-running the optos or it's locking up.  I haven't looked to be sure. 

I still haven't looked into the distortion it's producing.  I need to put it back on the scope to be sure, but I'm assuming it's generating a hot enough signal to overdrive the two amps I've tried it on.  I'll update everyone when I figure it out. 

Other than those few expected bumps, what a great effect!  Besides the standard phasing sounds, the Envelope Follower is really effective.  Very quacky.  Crank the EF and add some Envelope Modulation for a very usable delayed vibrato.  Set the Feedback to near oscillation and the Modulated EF gets pretty trippy.

Thanks again, Paul.  It took me way too long to get it soldered up, but it's very nice to have!